Company working hours

The working hours of many Japanese companies are from 9 a.m. to 5 p.m. Recently, however, the “flexible time” system has been spreading, in which workers have a certain degree of freedom to fix their working hours themselves. The “flex system” is popular because it means workers can avoid the worst of the commuting rush and work according to their life pattern.
On the other hand, it’s also true that many people feel bad about leaving earlier than their colleagues or bosses, even if their workload for the day has been completed. That’s why you shouldn’t forget to show careful consideration to colleagues. If you do leave before them, it’s a good idea to say O-saki ni shitsurei shimasu before you go. It means “I’m very sorry to leave before you!”
source: http://www3.nhk.or.jp/lesson/english/tips/9.html